1. Field of the Invention
The present invention relates to an electron-beam size measuring apparatus and a size measuring method with electron beams for inspecting a sample by irradiating an electron beam onto the sample.
2. Description of the Prior Art
During processing of manufacturing a semiconductor device, an inspection of a sample, a measurement of a pattern line width and the like are carried out with an electron beam apparatus such as an electron microscope. In an inspection or measurement of a sample, a portion to be inspected is scanned while being irradiated with an electron beam, and is displayed as an image on a display device by converting the amount of secondary electrons or the like to luminance.
When a sample is inspected or measured in this way, the sample is irradiated with an electron beam. This irradiation with the electron beam brings about a phenomenon in which a surface of the sample becomes charged. More specifically, the irradiated surface becomes charged positively or negatively by a difference in the amount of charge between charged particles entering the sample and charged particles emitted from the sample. When the surface of the sample is charged, emitted secondary electrons are accelerated or drawn back to the, sample, and thus the efficiency of secondary electron emission is changed. This consequently produces a problem that image quality of the sample surface becomes unstable. In addition, when the charging of the sample surface progresses, the primary electron beam may be deflected, thereby causing a distortion in an image, in some cases.
To solve such problems, various methods for preventing a sample surface from being charged have been proposed.
As a technique relating to this, Japanese Patent Application Laid-open Publication No. 2003-142019 discloses a method for controlling charging of a sample surface by using an accelerating voltage making a secondary electron yield more than 1 and an accelerating voltage making the secondary electron yield less than 1. Moreover, Japanese Patent Application Laid-open Publication No. 2006-54094 discloses a method for preventing the potential of a sample surface from becoming positive in a way that a voltage from an electrode disposed right above the sample pushes back secondary electrons emitted from the sample surface.
As described above, there occurs a phenomenon in which a sample becomes charged during an inspection of the sample with an electron beam apparatus. However, in a case where a sample can be electrically connected like a wafer, for example, there are no particular problems because a charging phenomenon of the sample can be prevented by grounding a conductor on the wafer electrically connected.
However, a charging phenomenon of a sample occurs in a case where the sample is nonconductive, or where the sample is electrically floating without being grounded even though the sample is made of a conductive material.
For example, in a case of measuring a dimension of a photo mask, which is to be used as a master for exposing a semiconductor, charging occurs in the following two statuses. In a first status, a conductor such as chrome is disposed over an entire surface of a glass substrate during processing of fabricating a wiring, and is covered with a resist wiring for etching the chrome to form the wiring. In a second status, the wiring made of the conductor such as chrome is disposed on the glass substrate after completion of the processing of fabricating the wiring.
The layer made of the conductor such as chrome is disposed over the entire surface of the glass substrate immediately before the chrome is etched, in particular. For this reason, when a certain portion becomes charged due to irradiation with an electron beam, the conductive layer over the entire surface of the substrate is charged. This consequently influences an inspection or dimensional measurement of another portion. In addition, even when charging at one portion is small, charging eventually becomes large by irradiating several hundreds to several thousands portions with the electron beam, which makes it impossible to figure out a dimensional relationship between a dimension measured first and a dimension measured last.
In regard to such a change in measured dimension, an experiment of the present inventor revealed that, in a case where the irradiation energy of an electron beam is 1500 (eV), for example, a change of sample surface potential of 10 (V) causes a dimension of 2 (μm) to vary by 6 (nm).
Compared with this, it is required that an electron-beam size measuring apparatus should have dimensional accuracy within an error range of not more than 1 (nm) when measuring a dimension of 2 (μm). Accordingly, in the case of the irradiation energy of 1500 (eV), an allowable range of a change of sample surface potential is on the order of 1.7 (V).
In addition, in a case of dimensional measurement of a resist material used during processing of forming wiring, the irradiation energy of an electron beam is generally lowered down to approximately 500 (eV) for the purpose of avoiding a damage of the resist material. In this case, the change of sample surface potential must be set equal to or less than approximately 0.6 (V) in order to satisfy the requirement for the dimensional accuracy.
Moreover, since some types of resist materials have high damage sensitivity to an electron beam, the irradiation energy must be lowered down to approximately 400 (eV) to 300 (eV), and it is desirable that a change of sample surface potential be close to 0 (V) as much as possible.
Note that it is difficult to maintain sample surface potential fixed with accuracy of not more than 1 (V) with a method for controlling charging of a sample surface by using secondary electrons.